When a vehicle shifts gears while accelerating, for example, an output of a transmission continues to increase in speed while the input to the transmission is able to reduce speed, allowing additional torque to be input into the transmission. For example, in a conventional non-hybrid vehicle, the engine speed can change from 6000 rpms to 2000 rpms due to a gear shift. With such a speed drop, a sudden torque bump is provided by the engine. If the torque bump is large enough, and the momentum and inertia of the spinning engine are also large enough, the operator of the vehicle may experience a perceivable “shift shock.”
In certain types of hybrid electric vehicles, an electric motor may also selectively provide torque to the input of the transmission. If the motor is coupled to the engine such that they rotate together when providing torque throughout the gear shift, the motor presents even more factors that impact the torque bump and shift shock.